1. Field of the Invention
The present invention relates to the design of integrated circuits, and more specifically to a method and apparatus for measuring power of analog signals.
2. Related Art
Integrated circuits are often designed to process analog signals. For example, a cell phone operating using GSM or CDMA may contain integrated circuits to amplify analog signals representing voice, and transmit voice encoded signals at radio frequencies (greater than 800 MHz).
It is often desirable to measure the power of analog signals. Power generally refers to the strength of the analog signals, and the measurement may be used in several ways. For example, it may be desirable to measure the power of analog signals in an integrated circuit such that the strength of the eventual signals transmitted may be controlled to be within a desired range. Such control provides several benefits, for example, protection of devices from damage due to extra power, optimizing power consumption, etc.
Power of an analog signal may be measured by root mean square (RMS) of the voltage. Various approaches are known in the prior art to measure the power of the analog signals. In one prior approach, diodes or transistors are used to rectify an input analog signal and a filter is used to average the rectified signal. The output of the filter represents the power level of the input analog input signal.
In general, the accuracy of representation of power level at the output of the filter depends on the extent of ideal characteristics exhibited by the components (rectifiers and filters) exhibiting the corresponding. However, components generally deviate from ideal characteristics, and the degree of variation depends on various factors such as ambient temperatures, imperfections in manufacturing processes, etc.
For example, a rectifier (diode or transistors, in the above description) may exhibit non-linear characteristics, and pass through both positive and negative peaks of the low amplitude input signals, resultingin improper rectification. As an illustration, if both positive and negative peaks are passed through, the average of the rectified signal would equal zero, thereby not representing the correct power level.
Even for high amplitude signals, a part of negative peak may pass through such a rectifier circuit, which also results in improper averaging of the rectified signal. At least due to the erroneous results caused, such an approach may be undesirable in several environments.
In an alternative approach, the signals with low amplitude are amplified to a desired amplitude level prior to rectification such that only positive peaks are passed through and at least most of the negative peaks are not passed through a rectifier circuit. One problem with such an approach is that the cost and board space (assuming the integrated circuit is placed on a board) are increased due to the pre-amplification requirement.